The Iraq War, also called Second Persian Gulf War, was a conflict in Iraq that consisted of two phases. The first of these was a brief, conventionally fought war in March–April 2003, in which a combined force of troops from the United States and Great Britain (with smaller contingents from several other countries) invaded Iraq and rapidly defeated Iraqi military and paramilitary forces. It was followed by a longer second phase in which a U.S.-led occupation of Iraq was opposed by an insurgency. After violence began to decline in 2007, the United States gradually reduced its military presence in Iraq, formally completing its withdrawal in December 2011.